It is well known in the art that acyclic conjugated alkadienes, as well as vinyl-substituted aromatic monomers as such or in admixture with each other, can be polymerized utilizing an alkali metal initiator system. Various homopolymers, random or tapered copolymers, as well as block copolymers with a wide scale of properties have been produced by this polymerization system.
It is also known in the art that alkali metal-terminated polymers obtained by such a polymerization process, such as lithium-terminated polymers, can be coupled in order to produce polymers with specific properties. In accordance with this known process, an alkali metal-terminated polymer is treated with a compound having two or more functional groups containing two or more reactive sites capable of reacting with the carbon-alkali metal bonds of the alkali metal-terminated polymer. A multifunctional coupling agent thereby becomes a nucleus for the resulting structure. From this nucleus long-chain polymer branches radiate, and such coupled polymers have specific properties that render them useful for particular applications.
Coupled polymers that are derived from coupling agents possessing three or more reactive sites are frequently called radial polymers. Such radial polymers have been of particular interest because of their increased Mooney viscosity, improved processability, and reduced cold flow as compared to the uncoupled or parent polymers. Hydrogenation of the alkadiene-based radial polymers has received considerable attention in recent years because the hydrogenation of such polymers increases the resistance of such polymers to environmental attack, e.g., oxygen and/or ozone deterioration.
It has, however, been found that some of these radial polymers are not sufficiently stable under hydrogenation conditions. More specifically, it has been found that radial polymers that have a connection between an inorganic atom and the first carbon atom of the parent polymer chain sometimes are not as stable as radial polymers in which the connection between the coupling agent and the polymer is a carbon-carbon bond. If this cleavage of the bond between the inorganic atom of the coupling agent and the carbon atom of the polymer chain occurs, the hydrogenated radial polymer is admixed with hydrogenated but no longer coupled polymer chains. This effect can impair the properties of the coupled polymer. Therefore, it would be desirable to have a coupled polymer which is formed from a coupling agent which does not contain inorganic atoms connected to its active sites and at the same time provides a high degree of coupling efficiency.